The following simple calculator will show the economic payback period (in years) for a hybrid/non-hybrid vehicle pair. For each vehicle, enter its cost (equipping both to be as near identical as possible) and combined MPG from fuel economy.gov. Also enter values for how many miles you drive each year and the average price of gas, and you'll see the payback on your chosen vehicle.
Once you've put the data in, you can change any of the values (miles and gas cost, for instance) to see their impact on the payback period.
(Fields marked with an asterisk are required.)